Remove <meta> generator tag
Remove <meta> generator tag

This reverts 8a1a3abe9b1a29dddae26c9f75b8c9b27f0409d5. Maintaining the
version number is a bit of a pain. Also, I am not aware of any use for
this tag.

It looks like there was some difficulty with getting the right version
string into this tag in the past, addressed by
ac1238282fa9e4b74d8dea0845f54d6361326ddd. However, that is no longer a
problem in the merged odoc repo.

ocaml-doc/odoc,ocaml-doc/odoc